Adaptive modulation and coding is a key enabling concept and technology for high-speed wireless data transmission. A wireless channel is typically a random fading channel. Adaptive coding and modulation is a commonly employed solution for transmitting data over such an unknown channel. Conventional design methodology provides a large fade margin in the transmit signal power to combat deep fades which may occur. Such fade margins are typically at least 6 dB, which represents a 200-300% throughput loss. The aim of adaptive coding and modulation is to fully utilize the channel capacity and to minimize the need to use such a fade margin by dynamically selecting the best coding and modulation configuration on-the-fly. This requires the transmitter to have accurate information about the instantaneous channel quality. Such instantaneous channel quality information is extracted at the receiver and fed back to the transmitter. The conventional approach is to measure the channel (signal) to interference power ratio (CIR) at the receiver front-end. Based on the instantaneous CIR and a targeted performance, the transmitter determines and applies the appropriate coding rate and modulation. In general, due to a complex propagation environment, a fast and accurate measurement of the CIR is a very difficult task.
Conventional channel quality measurements can be classified into two categories: (1) pilot based channel quality measurements and (2) decision feedback based channel quality measurements. These methods use the correlation of known sequences, typically Pseudo-Noise (PN) codes, with both the desired signal and the interference. For a slowly varying channel with a sufficient measurement time, the conventional methods can provide an accurate CIR measurement.
Referring to FIG. 1, a conventional pilot based CIR estimation scheme will now be described. In the context of MIMO-OFDM (Multiple Input Multiple Output—Orthogonal Frequency Division Multiplexing), the conventional channel quality measurement uses a pilot header containing two identical known OFDM symbols upon which to base an indication of the current channel quality. FIG. 1 shows a first, second and third base transceiver station (BTS) 100, 110, and 120 transmitting their respective signals, and a mobile station 130 receiving these signals. Mobile station 130 is configured to receive, demodulate and decode a signal transmitted by the second base transceiver station 110. The signals transmitted by the first base transceiver station 100 and the third base transceiver station 120 are received as interference by the mobile station 130. A channel associated with the signal having received signal power C transmitted by base transceiver station 2 (BTS2) 110 is the channel whose quality is to be measured. Suppose that we have N PN codes, and that the length of each PN code is N chips, we then have:PNi·PNj≈0 i≠jPNi·PNi=N 1≦i≦N.This important relation that the PN codes form a near orthogonal set allows for the extraction of specific channels using the Pilot channel PN codes. In FIG. 1 only three BTSs are shown, and hence there are only three PN codes. The second BTS 110 encodes a signal whose associated channel quality is to be measured, at ENCODER-2 112. The encoded signal is modulated using a PN Code which here is labelled Pilot-PN2 114 before eventually being transmitted through an antenna 118 to the mobile station 130. The first BTS 100 encodes a signal, which appears as a first interference signal to the mobile station 130, at ENCODER-1 102. This encoded signal is modulated using a PN Code Pilot-PN1 104 before eventually being transmitted through an antenna 108. The third BTS 120 encodes a signal, which appears as a second interference signal to the mobile station 130, at ENCODER-3 122. This encoded signal is modulated using a PN Code which here is labelled Pilot-PN3 124 before eventually being transmitted through an antenna 128. All three signals transmitted by antennas 108, 118, and 128 are received by the mobile station 130 at the receiver front-end 134 through antenna 132. The received signal is then passed to a decoder 138 for extraction of the channel to be recovered. The received signal is also passed on to a first correlator 140, a second correlator 142, and a third correlator 144. The correlators of FIG. 1, perform sub-operations corresponding to multiplication, summation, and absolute-value-squared, effectively performing an operation corresponding to taking an inner product of two inputs. The first correlator 140 performs a correlation between the received signal and the PN code Pilot-PN1, which was used to modulate the signal appearing to the mobile as the first interference signal, and outputs an interference power I1. The second correlator 142 performs a correlation between the signal and the PN code Pilot-PN2, which was used to modulate the signal whose quality is to be measured, and outputs a signal power C. The third correlator 144 performs a correlation between the received signal and the PN code Pilot-PN3, which was used to modulate the signal appearing to the mobile as the second interference signal, and outputs an interference power I2. A calculating operation 150 computes the CIR which in this case is simply C/(I1+I2).
In general, this approach can be applied to M base transceiver stations. Let BTSi (1≦i≦M) be the M adjacent base transceiver stations, Ei be the corresponding energy from the ith base station that is measured at the mobile station 130, let S be the combined total signal energy received by the mobile at receiver front-end 134, and let BTS2 be the base transceiver station whose associated CIR is to be measured, then
            C      =                                    max                          1              ≤              i              ≤              M                                ⁢                      (                                          S                ·                P                            ⁢                                                          ⁢                              N                i                                      )                          =                              E            2                    ·          N                      ,          a      ⁢                          ⁢      n      ⁢                          ⁢      d            I    =                            ∑                      i            ≠            2                          ⁢                  (                                    S              ·              P                        ⁢                                                  ⁢                          N              i                                )                    =              N        ·                              ∑                          i              ≠              2                                ⁢                                    E              i                        .                              In these equations C and I are energies although for the purposes of determining the ratio C/I, either energy or power may be used. Since the pilot header is composed of two identical OFDM symbols, the CIR calculation process can be based on the average over the two symbols, thus reducing noise. These methods, however, fail to work if the channel is a multi-path fading channel and/or mobility speed is high. One solution is to insert more pilots to improve the measurement quality, however, this introduces overhead which significantly reduces spectral efficiency. For example, in 2G and 3G wireless systems, the pilot overhead is about 20-35%, and the pilot design for these systems is not suitable for fast channel quality measurement. This is the case because fundamentally the accuracy of the channel quality measurement is limited by the Cramer-Rao lower bound, which implies that the accuracy of channel measurement can be gained only at the expense of more pilot overhead (either in time or in power).
As an example of this trade-off, in a proposed MIMO-OFDM system, a pilot header is transmitted every OFDM frame in 10 ms (15 slots). To facilitate adaptive modulation in the mobility case, a CIR estimation must be fed back to the BTS every 2 ms (3 slots). Therefore, CIR measurement based on a pilot header can not provide accurate instantaneous channel quality information. If the actual CIR does not change significantly during that 10 ms, then by measuring the energy of the pilots, one may roughly track the CIR. However, by doing so, the accuracy may diminish towards the end of the slot, as the assumption that the interference is a constant becomes more and more inaccurate.
The above discussed channel quality measurement is for adaptive coding and modulation, and does not in any way relate to channel estimation.
Channel quality measurement is a different concept from channel estimation. Channel quality measurement is performed to measure the channel quality so that proper coding and modulation set can be chosen. Channel estimation is performed to estimate the channel response so that coherent detection can be implemented.
In some wireless communication systems that employ Orthogonal Frequency Division Multiplexing (OFDM), a transmitter transmits data symbols to a receiver as OFDM frames in a MIMO (multiple input, multiple output) context. One of the key advantages of MIMO-OFDM systems is its ability to deliver high-speed data over a multi-path fading channel, by using higher QAM size, water pouring and/or adaptive modulation. In the MIMO-OFDM system, there are two major design challenges: (1) To combat high Doppler spread and fast fading due to high speed mobility (2) To provide a common fast signalling channel to realize fast physical and MAC layer adaptation signalling. To solve the mobility problem, a pilot channel is commonly used in OFDM design; such a pilot channel can be optimized by using the scattered (in time and frequency) pilot pattern. The common fast signalling channel design must be sufficiently reliable to allow most of mobiles to detect the signalling, which introduces a significant amount of system and spectral overhead to sustain the signalling throughput. In the conventional OFDM design scattered pilot and fast signalling channel are arranged as separate overhead channels.
The phase and amplitude of the data symbols may be altered during propagation along a channel, due to the impairment of the channel. The channel response may vary with time and frequency. In order to allow the receiver to estimate the channel response, pilot symbols are scattered among the data symbols within the OFDM frame. The receiver compares the values of the received pilot symbols with the known transmitted values of the pilot symbols, estimates the channel response at the frequencies and times of the pilot symbols, and interpolates the estimated channel responses to estimate the channel response at the frequencies and times of the data symbols.
Transmit Parameter Signalling (TPS) symbols are also transmitted with the data symbols. The TPS symbols are transmitted over specified sub-carriers within the OFDM frame, and are used to provide common signalling channels to allow fast physical and media access control layer adaptation signalling.
Both the pilot symbols and the TPS symbols are overhead, in that they do not carry data. In order to improve the data rate of an OFDM communication system, the overhead within the OFDM frames should be minimized. The minimization of overhead is particularly important in Multiple-Input Multiple-Output (MIMO) OFDM systems. In a MIMO OFDM system having M transmitting antennae and N receiving antennae, the signal will propagate over M×N channels and there may be up to M sets of pilot symbols in the overhead. An example of an OFDM frame format with dedicated TPS and pilot channels is shown in FIG. 7 for the single input, single output case. The horizontal axis 704 shows a circle representing the frequency of each of a plurality of OFDM sub-carriers. The vertical axis 706 is time, with each row representing an OFDM symbol. A set of OFDM symbols constitutes an OFDM frame. In this example, the pilot channel is transmitted in a scattered manner, with the pilot symbols being transmitted every third sub-carrier, and for each sub-carrier every sixth frame. Thus, the first sub-carrier 700 has pilot symbols 701 in the first, seventh (and so on) OFDM symbols. The fourth sub-carrier 702 has pilot symbols 705 in the fourth, tenth (and so on) OFDM symbols. In addition, the third, ninth, 15th, and 21st sub-carriers of every OFDM symbol are used to transmit TPS symbols, collectively indicated at 708. The remaining capacity is used for traffic.